1. Field of the Invention
The present invention relates to a method of generating a photolithography patterning device, a computer program, a patterning device, a method of determining the position of a target image on or proximate a substrate, a measurement device, and a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to accurately apply a desired pattern onto a target portion of a substrate, the reticle should be aligned with respect to the substrate. Therefore, according to the prior art, the relative position of the reticle with respect to the substrate is set accurately, by measuring and adjusting the relative position. Alignment of the patterning device with respect to the substrate is, according to the state of the art, done using two alignment actions.
In the first action the substrate is aligned with respect to the substrate stage carrying the substrate, while in the second action the reticle is aligned with respect to the substrate stage. As a result of these two actions, the reticle is aligned with respect to the substrate, as desired.
In case a single stage machine is used, the first and second action are carried out at the exposure position. In case a dual stage machine is used, the first action may be carried out at a first position, remote from the exposure position. Then, the substrate stage with the substrate positioned on it is transported to the exposure position, where the second action is performed.
The first action may be carried out with two sensors. A first sensor measures the relative position of the substrate with respect to the substrate stage in X, Y and Rz directions, where the XY plane is defined as the plane that is substantially parallel with the surface of the substrate, the X- and Y-direction being substantially perpendicular with respect to each other. The Z-direction is substantially perpendicular with respect to the X- and Y-directions, so Rz represents a rotation in the XY plane, about the Z-direction. A more detailed description about this sensor is for instance provided in U.S. Pat. No. 6,297,876. A second sensor, usually referred to as the level sensor, measures the height of the substrate surface in dependence on locations on the substrate to be exposed, creating a height map based on the determined heights, and also determines the rotations about the X and Y axes: Rx, Ry.
Next, in the second action, the reticle is aligned with respect to the substrate stage. This may be done with an image sensor, such as a transmission image sensor (TIS), as will be known to a person skilled in the art. A TIS measurement is performed by imaging a first alignment pattern (mask alignment mark) provided on the reticle through the projection system (lens) to a second alignment pattern provided on the substrate stage. The alignment patterns may include a number of isolated lines. Inside the substrate stage, behind the second alignment pattern a light sensitive sensor is provided, e.g. a diode, that measures the light intensity of the imaged first alignment pattern. When the projected image of the first alignment pattern exactly matches the second alignment pattern, the sensor measures a maximum intensity. The substrate stage is now moved in the X- and Y-directions on different Z-levels, while the sensor measures the intensity. Therefore, the TIS is actually an aerial image sensor, in which multiple scanning slits probe the aerial image of isolated lines. Based on these measurements, an optimal relative position of the substrate stage can be determined. A typical TIS sensor will be explained in further detail below with reference to FIG. 14. It will be understood that instead of a transmission image sensor, also a reflective image sensor may be used. In this case the second alignment pattern provided on the substrate stage is reflective, and the light sensitive sensor is not positioned inside the wafer stage. Therefore it will be understood that although the text refers to transmission image sensors, this may in general be any type of image sensor.